The copending U.S. application of Richard T. Ingwall and Mark A. Troll, Ser. No. 175,208 filed Mar. 30, 1988 (corresponding to PCT Published Application WO 88/04796 published Jun. 30, 1988) now U.S. Pat. No. 4,970,129 issued Nov. 13, 1990, discloses and claims volume phase holograms characterized by the fact that microvoids between the fringes of the recorded interference pattern are at least partially filled with a material having an index of refraction different from that of the matrix. These void-filled holograms may be referred to as composite holograms. The material filling the microvoids changes the properties of the holographic element, e.g., it may provide enhanced stability, or it may modify the optical properties by changing the average refractive index. The nature and properties of such composite holograms have been described by R. T. Ingwall and M. A. Troll in "Mechanism of Hologram Formation in DMP-128 Photopolymer", Optical Engineering, Vol. 28, No. 6, pp. 586-591 (June 1989).
It is known to form a polymer film having a liquid crystal dispersed therein; see, for example, H. G. Craighead et al, "New Display Based On Electrically Induced Index Matching In An Inhomogeneous Medium", Appl. Phys. Lett., Vol. 40, No. 1, pp. 22-24 (1 Jan. 1982), and Bao-Gang Wu et al, "Angular Discrimination of Light Transmission Through Polymer-Dispersed Liquid-Crystal Films", J. Appl. Phys., Vol. 62, No. 1, pp. 3925-3931 (1 Nov. 1987). Such polymer-dispersed liquid crystal films may be prepared, e.g., by polymerization-induced phase separation or by thermally-induced phase separation. In either process, a homogeneous solution containing the liquid crystal and the polymer or polymer precursor is converted into a film or polymer matrix having liquid crystal droplets homogeneously dispersed therein. The utility of such films is based upon light scattering.
It also has been proposed to modulate a hologram by providing a liquid crystal in the surface grooves of a holographic diffraction grating. See Simeon Sainov et al, "Halographic Diffraction Grating Controlled by Means of Nematic Liquid Crystal", Mol. Cryst. Liq. Cryst., Vol. 152, pp. 609-615 (1987).